The invention relates to bar code scanning, and more particularly to a threshold control for interpreting the signal from a bar code scanner having a floating and decaying threshold.
In bar code reading, a reflected light signal is received and interpreted by the reading apparatus when the scanner is on. Intensity of the light signal varies, corresponding to light and dark (or white and black) elements being scanned. Generally, the bar code scanners encounter many different types of lighting conditions in the reading of bar codes, including wide variations in the maximum reflected light received under different conditions and also wide variations in the contrast between the reflected signal received from a black bar and that received from a white bar. A decision must be made as to whether a particular signal level represents black or white when interpreting the signal.
Therefore, bar code reading systems have sometimes included circuitry adapted to apply a floating threshold in the interpretation of black versus white. A typical system might use the derivative of signal strength with respect to time to detect transitions from black to white or white to black. For example, the circuitry might look at the maxima of the derivative signal received, then apply a certain percentage of the maximum positive or negative derivative signal value, to establish a positive or negative derivative signal level threshold. A derivative signal above the positive threshold or below the negative threshold will indicate a space-bar or bar-space transition. The specific circuit implementation determines which transition a positive signal represents.
Once selected in this way, the threshold should not remain fixed because the scanner almost always encounters different conditions which will vary the strength of the received signals. These differing conditions include changes in ambient light, glare, angle of approach, distance to the bar code, relative velocity of bar code and scanning spot (in the case of a flying spot scanner), varying reflectivity of the material being scanned, component drift, and variation in the power of the laser source.
Therefore, in many beam scanner bar code readers, the threshold is continually being adjusted while the scanner is on.
In a period of time when the scanner does not see a bar code, few if any strong transitions in the strength of the signal will be seen. Since the interpretive circuitry does not recognize whether this is due to the absence of a bar code or due to a change in conditions resulting in lower signal strength and contrast, the thresholds are normally caused to automatically "decay" over time, i.e. to become progressively lower so that signal transitions will again be picked up by the reading circuitry. The time constant of the decay is on the same order as the time to scan a few elements of the bar code. One method of implementing the decaying threshold is to represent the threshold value as a voltage on a capacitor, discharging through a resistor to provide a gradual decay. Such a decaying threshold will tend to produce perceived black/white transitions when the beam is scanned over plain paper, for example. Characteristics of the surface of the paper and the grain of the paper result in varying reflectivity which, even though at very low contrast, will appear to be bar code transitions when the threshold has decayed to a very low value. Thus, what is really noise can appear to be black/white transitions from a bar code. There has been a problem in the prior art, of compensating for or overcoming the condition of extraneous noise being perceived as black/white transitions.
A principal problem is that bar code reading is not performed efficiently if the reading mechanism spends an unnecessary amount of time trying to read noise. A second problem is the possible apparent loss of the le during white space before the bar code, the lack of which makes reliable decoding more difficult.
There have been several approaches to this problem. In one approach, a fixed bottom level was established for a black/white reading threshold, to guarantee that the decaying threshold could not become low enough to read noise such as that resulting from low contrasts in the grain or surface texture of paper. This was only a limited solution, since there is a need for the threshold to go quite low when a scanner is used at a relatively great distance from the label, or with low contrast labels. Wide differences in the distance at which the gun is used necessitate a wide range of possible thresholds.
A different approach also encompassed by the prior art was the use of a dynamic threshold in a hand held scanner, coupled with an analog delay line. Bar code readers using this approach included a special integrated circuit to delay the signal in one circuit, while using the signal instantaneously in another circuit. In the delay circuit, the signal was delayed for several milliseconds before a threshold was applied to it. Meanwhile, in the other circuit the signal was used instantaneously for establishing the threshold, using, for example, a fixed percentage of the peak derivative signal strength.
These analog delay line systems are effective in preserving the leading white space before the label, but not in removing noise elsewhere.
It is among the objects of the present invention to provide a relatively simple solution to the problem of noise pickup with scanner threshold decay, by utilizing some knowledge of the characteristics of a bar code signal and the characteristics of noise signals, to discern the difference between them and to raise the black/white signal threshold so long as noise continues to be detected.